1. Field of the Invention.
This invention relates to wood burning stoves in which heat from burning wood or similar combustible material is used to heat the air in a room or the like. More particularly, the invention relates to such stoves in which hot gases from a fire heat the walls of passage ways through which ambient air passes, the ambient air thereby being heated, and in which the flow path of the hot gases increases their contact with the passage way walls to increase the amount of heat transferred to the ambient air. In some respects, this invention is an improvement over the invention described and claimed in my prior U.S. patent application Ser. No. 855,932, now U.S. Pat. No. 4,127,100.
2. Description of the Prior Art.
Wood burning stoves, so named because wood is the principal fuel used with them, have existed for many years . Most such stoves are of generally rectangular box-like shape and are fabricated from iron or steel plate. Common to virtually all wood burning stoves are a fire chamber or fire box in which the wood is placed for burning; draft inlet means to admit air to the fire chamber for combustion of the wood; and a flue or smoke stack to allow hot gases and fumes from the fire to escape from the fire chamber. In most stoves the fire chamber includes a hearth area lined with firebrick to support the burning wood. The wood is placed either directly on the brick or on a grate spaced above the brick. Additionally, some such stoves include controls on the draft inlet means which allow the amount of air entering the fire chamber to be decreased to a level which will just support combustion, thereby providing longer burning times for each load of wood burned.
Prior wood burning stoves have been designed in a variety of ways; most early designs of such stoves function as both cooking and heating stoves--that is, they include flat top surfaces for cooking and/or ovens for baking, and when in use also provide heat by radiation from the hot metal surfaces to the air of the rooms where they are located.
To augment the radiant heat from wood burning stoves, prior workers have devised several methods for providing a flow of heated air from the stoves. In one approach, one or more outer walls are spaced from the fire chamber walls to form an air passage way separate from the fire chamber, the passage way having a lower inlet and an upper outlet for ambient room air. Air in the passage way is heated by contact with the hot fire chamber walls and rises through the outlet, thereby drawing additional air into the passage way. As the additional air is heated, it, too, rises; a low volume flow of heated air is thus established to help warm the room. This approach, while useful, is inefficient; i.e., more of the heat from the burning wood escapes through the flue than is extracted for heating the room.
Another more recent approach to providing a flow of heated air to a room from a wood burning stove described in U.S. Pat. No. 4,213,443 employs what is commonly known as a step stove, in which the stove top is divided into two horizontal portions, the rear portion being at a higher level than the front portion, with a short, nearly vertical section of plate joining the two. In this approach, air from the room flows, either by convection or through use of a blower, into a pipe mounted flush with the surface of the hearth floor and extending across the hearth at the front of the fire chamber. Two other pipes, one at each side of the hearth, communicate with the first pipe. Each of the other pipes extends from the front to the rear of the fire chamber, being positioned flush with the surface of the hearth floor; at the rear of the fire chamber each pipe makes a right-angle turn and extends vertically upward to a point near the top of the fire chamber, where it makes another right-angle turn and extends horizontally towards the front of the fire chamber, finally opening to the room at the short section joining the front and rear portions of the stove top. This stove has improved heating efficiency because the air pipes are within the fire chamber itself and thus extract more heat from the fire and hot gases rising from the fire than is extracted by the previously described passage way arrangement. Even here, however, a substantial amount of heat from the burning wood passes to the flue; that this is so is evidenced by the fact that the stove described can also heat water circulating through an optional coil which encircles the flue at the rear of the fire chamber.
Yet another approach of the prior art employs one or more horizontal or nearly horizontal pipes within the fire chamber and above the hearth. The pipes are heated by the hot gases rising from a fire on the hearth. Cool ambient air enters the hot pipes from an air collecting chamber outside the fire chamber at the rear of the stove and is heated as it passes through the pipes; the heated air leaves the pipes at the front of the stove, where it helps to heat the room. In all such stoves of which I am aware, the air flow is due exclusively to the air entering the pipes tending to rise as it is heated and thereby pulling additional cold air in from the rear of the pipe to establish a low volume air flow into the room. In some of these stoves nearly horizontal baffle plates extend partially across the fire chamber to direct hot gases from the fire into more intimate contact with the pipes. These stoves provide somewhat more efficient extraction of heat from the burning wood than the first above described stoves, but they are still basically inefficient; some increase in efficiency can be gained by use of a large number of small diameter pipes such as shown in U.S. Pat. No. 585,027, in which 14 small air pipes are used, but then the stove becomes complex and unduly expensive to fabricate.
From the foregoing, it is evident that wood burning stoves of the prior art, while useful to some extent in heating ambient room air, are not highly efficient in utilizing the heat from the burning wood for that purpose. Consequently, more wood is required to heat a given room with prior art stoves than would be necessary with better efficiency.
As set forth in my prior U.S. patent application Ser. No. 855,932, now U.S. Pat. No. 4,127,100 I found that by positioning an air chamber and air pipes within the fire chamber of a wood burning stove, passing ambient air through such chamber and pipes and suitably baffling the flow of hot gases from the fire in the stove so that such gases contact substantially all of the surfaces of the chamber and pipe, high efficiency heating of ambient air can be achieved in a wood burning stove. Because of such efficiency, less wood is required to heat a room and savings in fuel costs can thereby be realized.
In my prior application I provided a wood burning stove for providing heated air to a room or similar area, comprising an enclosed fire chamber including a generally horizontal floor at the bottom thereof, a forward portion of the floor comprising a hearth for receiving wood to be burned; draft inlet means at the front of the fire chamber; a flue at the rear of the fire chamber and near the top thereof; an enclosed air chamber positioned between the rear of the hearth and the rear wall of the fire chamber and extending generally vertically from the bottom of the fire chamber to a point spaced from the top of the fire chamber, the air chamber having front and rear walls, a bottom and a top, the bottom of the air chamber communicating with the ambient air; at least one air pipe extending generally horizontally within and along the top portion of the fire chamber, one end of the air pipe communicating with the interior of the air chamber at the top of the air chamber and the other end opening to the ambient air at the front of the stove; and a generally vertical baffle plate positioned between and spaced from the air chamber and the rear wall of the fire chamber and extending the full width of the interior of the fire chamber, the baffle plate being joined at its top edge to the top of the fire chamber, the bottom edge of the baffle plate being spaced from the bottom of the fire chamber, whereby the flow of hot gases rising from a fire in the fire chamber would be such that the gases contact the surface of the air pipe and both the front and rear walls of the air chamber.
The stove of my prior application functions quite well; as indicated in such application, with such stove I can heat a room for extended periods of time, as much as 24 hours, using a single log, and with proper adjustment of the draft inlet complete combustion of the logs can be achieved, i.e., very few ashes accumulate on the hearth. Also, because of the thorough contact of hot gases with the walls of the air pipes and air chamber, a very high proportion of the heat is extracted from such gases and transferred to the ambient air so that the gases escaping through the flue are at a relatively low temperature; thus, with my stove the danger of burns from a hot flue or flue pipe is less than with conventional stoves of the prior art.
I found, however, that my vertical air chamber-separate vertical baffle plate stove was somewhat complex to fabricate in smaller sizes and the separate vertical baffle plate added undesirable weight to large double-door models. I concluded that if the air chamber and baffle plate could be combined into one unit, the resulting stove would retain all the desirable features of my prior stove and at the same time overcome the just discussed difficulties encountered with that stove.